Wells, also known as wellbores or boreholes, are drilled to reach reservoirs of underground petroleum and other hydrocarbons. Often, wells are drilled in a vertical direction. The geological formations or strata that make up the earth's crust, however, generally lie in horizontal layers, so vertical wells are substantially perpendicular to the strata. If a certain formation contains hydrocarbons, it is often desirable to steer the drill in the horizontal direction to keep the well bore within that formation (called the "pay zone"), thus maximizing the recovery. Because the formations are underground and thus hidden from view, the well operator usually does not know exactly where to drill. Steering also can be difficult since formations may dip or divert.
To aid the well operator in locating and identifying subterranean formations, a probe (or "sonde") may be lowered into the wellbore to collect information about the structure of the formations, a procedure commonly known as "logging." The sonde typically includes one or more sensors to measure parameters downhole, is constructed as a hermetically sealed cylinder for housing the sensors, and hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. Normally, a cable within the sonde supplies operating power to the sonde and transmits information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
The information collected by the sonde provides insight into the composition of the formations, including whether or not the formations are likely to contain hydrocarbons. Geological formations must be sufficiently porous to contain hydrocarbons, for example, so porosity of the strata is often measured to determine the capability of the formation to store hydrocarbons. Saturation of the formations is often measured, as well, to determine the amount of water, hydrocarbon, or fluid stored in the porous formations. Fundamental properties such as porosity and saturation can be used to estimate important characteristics of the formation, such as the size and quality of a reservoir and the ability of the reservoir to flow through the formation into the borehole.
While wireline logging is useful in characterizing formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed or tripped from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically pays daily fees to rent the drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must at times make decisions (such as the direction to drill) possibly without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. Furthermore, wireline logging occurs a relatively long time after the wellbore is drilled, calling into question the accuracy of the wireline measurements. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling fluids invade the formation around the wellbore. In addition, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
To address the limitations associated with wireline logging, special tools were developed to collect data during the drilling process. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections "real-time" to optimize drilling performance. With a steerable system, the driller may change the direction of the drill bit. By detecting the adjacent bed boundaries, adjustments can be made to keep the drill bit in an oil rich pay zone. Moreover, measuring formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Making formation and borehole measurements during drilling also can save valuable rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole and the movement and location of the drilling assembly during drilling are known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, are commonly known as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well.
Because hydrocarbon-bearing formations tend to have unique and identifiable electrical properties, one type of logging, generally known as electric logging, measures these electrical properties. One of these electrical properties, known as conductivity, is a measure of how readily the formation conducts electric current. Conductivity, and its reciprocal property, resistivity, provide insight into formation characteristics such as fluid saturation, net reservoir thickness, porosity, and structural or stratigraphic dip. Measuring resistivity or conductivity is generally known as resistivity logging and is achieved by measuring electrical potentials, and sometimes currents, and/or electromagnetic waves in the borehole. These measured potentials, currents, and electromagnetic waves are influenced by the resistivities of all the materials surrounding the borehole.
Resistivity logging generally involves sending an electromagnetic wave from a transmitter on the LWD tool and capturing the wave at a receiver which is at another location on the LWD tool. For this reason, this type of resistivity logging is also known as electromagnetic wave logging. Typically, the transmitter sends the waves at a frequency between one and two million cycles per second (or 1-2 megahertz). Some tools, however, utilize frequencies in the range of thousands of cycles per second, or kilohertz. The formation resistivity causes changes in the intensity and timing of the transmitted wave, so the receiver does not receive an exact copy of the wave that the transmitter sent. Instead, the resistivity of the formation reduces (or "attenuates") the intensity of the signal and causes a time delay (or "phase shift") in the signal. Accordingly, the attenuation and phase shift can be measured at the receiver and used to gauge the resistivity of the formation, providing insight into the formation characteristics as described above. Resistivity derived from attenuation measurements is commonly called "attenuation resistivity," and resistivity derived from phase measurements is commonly known as "phase resistivity."
In a type of formation called "shaley-sand," for example, the shale bed can have a resistivity of about 1 ohm-meter. A bed of oil-saturated sandstone, on the other hand, is likely to have a higher resistivity of about 10 ohm-meters or more. The sudden change in resistivity at the boundary between beds of shale and sandstone can be used to locate these boundaries. In horizontal drilling, the drill bit can be steered to avoid this boundary and keep the wellbore inside the oil-producing bed.
A typical formation does not have a uniform (or "homogeneous") resistivity throughout, so it is usually desirable to measure the resistivity at various locations around the borehole to fully characterize the formation. Tools commonly measure the resistivity along a concentric ring around the borehole, at a radius which is called the "depth of investigation" or "radius of investigation." To thoroughly characterize the formation, measurements are taken at a variety of depths of investigation and at a variety of vertical positions within the borehole. The depth of investigation generally is determined by the distance between the transmitter and receiver, with a longer spacing resulting in a deeper depth of investigation and a shorter spacing providing a shallower depth of investigation. Accordingly, to measure the resistivity at multiple depths of investigation and thus achieve an accurate picture of the formation composition, a resistivity tool requires one transmitter/receiver pair for each desired depth of investigation. For instance, a resistivity tool that provides three depths of investigation requires three transmitter/receiver spacings. Such a resistivity tool might include, for example, one receiver and three transmitters or one transmitter and three receivers.
Because transmitter and receiver circuitry may respond differently to changing temperatures as a resistivity tool progresses downhole, early resistivity tools had to be calibrated across a range of temperatures. To maintain accuracy at certain intervals the calibration was repeated for each tool, requiring extra time, effort, and expense. More recently, compensated resistivity tools have been developed to overcome these biases. Compensated tools typically involve using extra transmitters and/or receivers which interact to cancel out the effects of the circuitry biases.
To measure the resistivity at a first depth of investigation, a first transmitter transmits a signal to one or more receivers. The geological formation attenuates and phase shifts the transmitted signal as it propagates to the receivers, so the received signals are slightly different than the transmitted signal in both magnitude and phase. A first compensating transmitter may transmit a compensating signal to the same set of receivers. The first compensating transmitter is spaced at the same distance from the receivers as the first transmitter but is located on the opposite side of the receivers. The signals received from both transmitters are combined using known techniques to produce a composite attenuation value and a composite phase value. For a given depth of investigation that is uniquely associated with the transmitter/receiver spacing, a resistivity value can be calculated as known function of either the composite attenuation value or the composite phase value.
To measure the resistivity at a second depth of investigation, a second transmitter must be located at a distance from the receivers that is different than the distance between the first transmitter and receivers (i.e., the distance must be less than or greater than the distance between the first transmitter and receivers). If the tool is compensated, then an additional compensating transmitter is required at the same distance from the receivers but on the opposite side from the main second transmitter. The receivers use signals received from the second transmitter and its associated compensating receiver to gauge the resistivity at a second depth of investigation, using either the composite attenuation value or composite phase value (or both) derived by combining the received signals.
To measure the resistivity at additional depths of investigation, the tool requires additional transmitters spaced at different distances from the receivers. Placing numerous transmitters on a resistivity tool, however, leads to some significant drawbacks. In particular, modern resistivity tools are fairly slow. The resistivity transmitters are fired sequentially, meaning that a greater number of transmitters results in a greater number of transmitter firings in any "set" of transmitter-receiver resistivity readings. Because firings occur sequentially, a large number of transmitters require a substantial amount of time to complete a set of resistivity readings. In addition, transmitters often are fired while the resistivity tool moves up or down the wellbore, and a large number of transmitters slow down the maximum practical speed at which a resistivity tool can progress through the borehole.
A less than ideal rate of movement up or down the borehole is not an insignificant problem. For example, in wireline logging, the sonde moves along the borehole wall as quickly as possible to minimize the time required to recover hydrocarbons and to minimize costs. As another example, in the LWD environment, a borehole may already be partially drilled and the drill bit assembly lowered a significant distance into the earth prior to actual drilling. An operator would like to quickly obtain a set of resistivity measurements while the frill string is being lowered downhole in the old wellbore. Further, in certain formations it is the data acquisition rate of the LWD tools, and not the ability of the drill bit to cut through formation, that limits drilling speed. Data acquisition while the drill bit assembly is being "tripped" or pulled up from the borehole is often also desirable. Because multiple transmitter resistivity tools use a long period of time to obtain a "set" of measurements, the disadvantages of a large number of transmitters undermine the advantage of a large number of transmitters.
Although substantial improvements have been made to resistivity tool design, numerous problems still exist. As explained, modern resistivity tools are slow and limit the maximum rate at which the tool may proceed past a wellbore wall. In addition, modern resistivity tools have high power requirements because of an increased number of transmitters and because transmitters far away from the receiver pair often transmit a stronger signal than transmitters close to the receiver pair. Lack of a sufficient number of transmitter/receiver spacings, however, limits the number of depths of investigation, thus preventing a thorough characterization of the formation.
For the foregoing reasons, a resistivity tool that does not require a different transmitter/receiver spacing for each depth of investigation would greatly improve the efficiency of logging while drilling operations. Despite the apparent advantages that such a system would provide, to date, no such device exists.